An electrophotographic photoreceptor has an electrically conductive substrate and a photosensitive layer formed thereon which includes an inorganic or organic photoconductor.
Recently, double layer photoreceptors with a combination of a charge-generation layer and a charge-transport layer have been shown to have higher sensitivity and some of them have been commercially employed. Those photoreceptors in which an organic material is used as a charge-transporting medium and which have high charge acceptance in addition to their higher sensitivity have been highly studied.
An electrophotographic photoreceptor is subjected to repeated copying operations which include charging by corona charging device, exposing, developing, transferring and cleaning steps and is required to have excellent durability in such repeated copying operations. When the aforementioned photoreceptors, especially those photoreceptors having an organic charge-transporting medium are subjected to the repeated copying operation, however, a remarkable lowering of the charged potential is observed and the resultant copies have poor qualities.
The lowering of the charged potential is believed to be due to various causes. One of the causes is the effect of the gases on the corona charging step. Active gases such as ozone, nitrogen oxides and the like produced in the negative or a.c. corona discharging are particularly a problem. For resolving the effect of the gases, the ventillation of the gases near the corona discharger was attempted. However, this was not successful since the gases cannot be completely removed.
As another method for resolving the effect of these gases, the addition of an anti-oxidant to the photosensitive layer, specifically for protection of the charge-transporting medium, has been proposed. The use of trialkyl phenol derivatives and dilauryl thiopropionate are mentioned in Japanese Patent Publication Nos. 50-33857 and 51-34736 and Japanese Patent Application Laying Open Nos. 56-130759 and 57-122444. However, the addition of these types of anti-oxidant is not sufficiently effective.
Now, there is a strong need for an electrophotographic photoreceptor having improved durability in repeated copying operations without showing the remarkable lowering of the charged potential and therefore the prolonged life.
An object of this invention is to provide such an electrophotographic photoreceptor having improved durability in repeated copying operations and therefore prolonged life.